The invention relates to a method for managing the operating range of an electric accumulator battery.
One field of application envisioned is in particular, but not exclusively, the management of lithium-ion batteries used in electric, hybrid or rechargeable hybrid vehicles. This type of battery comprises a plurality of electric accumulators or cells, including a rechargeable electrochemical system designed to supply a nominal voltage.
The operating range of the battery corresponds to the permissible range of state of charge for the battery, in terms of both charge and discharge. This operating range is characterized, on the one hand, by a maximum state of charge, corresponding to a state of charge above which the battery is not permitted to rise, and, on the other hand, by a minimum state of charge, below which the battery is not permitted to drop.
The maximum permissible state of charge is defined by the end-of-charge voltage or cutoff voltage, which may, for example, be measured across the terminals of the battery. This voltage is in effect representative of the charging limit of the cells that form the battery. In other words, it is the value that the maximum voltage across the terminals of each cell must reach at the end of charging in order to judge that charging has in fact finished. A high end-of-charge voltage increases the energy available at the start of life of the battery, but, in contrast, brings about faster ageing of the battery. Thus, in order to manage the end-of-charge voltage, it is necessary to find a compromise between the energy level provided by the battery in the short term and the durability of the battery.
It will be seen that the problem of managing the minimum state of charge of the battery is just as acute. In effect, if the minimum state of charge is too high, the energy available for the user will not be at the desired level with respect to a required minimum energy level, whereas, if it is too low, in certain instances of use, in particular in conditions involving cold temperatures, the battery risks being at a level of state of charge at which it will not be able to supply the required minimum power level. Thus, in order to manage the minimum permissible state of charge for the battery, it is also necessary to find a compromise between the desired energy level that one wishes to ensure for the user and the available discharge power of the battery over the entire operating range of the battery, including when cold.
In terms of performance, the battery system, consisting primarily of the cells and the dedicated BMS (Battery Management System) computer, must therefore ensure a required minimum energy level, not only at the start of life but also after a certain number of years, and a required minimum power level, not only at the start of life, over the entire operating range, but also after a certain number of years.
Patent document WO2012074406 discloses a method for managing the charging of a battery of an electric vehicle, wherein the BMS is able to determine a modifiable charging algorithm intended to be provided to the charger in order to ensure charging of the battery in such a way as to adapt the charging as a function of different conditions, such as temperature, electric network and type of charger. This method makes it possible to charge the battery to a desired energy level in nominal conditions. However, said method does not allow management of all of the causes of “dispersion” that can influence the available energy level. In effect, one of the difficulties lies in the fact that, for a predetermined end-of-charge voltage, the energy available for a user is not the same, in line primarily with three factors:                the temperature of the battery: thus, the lower the temperature of the battery, the smaller the amount of energy that can be discharged from this end-of-charge voltage, as the internal resistances of the cells are greater;        the ageing of the battery: the more degraded the state of health of the battery, the smaller the amount of energy that can be discharged from this end-of-charge voltage, as the capacity available (in A.h) in the battery is limited;        the cell imbalance, defined as being the difference between the state of charge of the cell having the highest charge and the state of charge of the cell having the lowest charge: thus, the greater the cell imbalance, the smaller the amount of energy that can be discharged from this end-of-charge voltage, because the lowest cell will reach its limit end-of-discharge voltage (minimum state of charge) earlier than the other cells.        